The study was initiated in September 1977 with 1000 children in the seventh grade of nine junior high schools located in Des Moines, Iowa, a community that has optimal amounts of fluoride in its supply of drinking water. The children were randomly assigned to one of the following three study groups: Group I (controls) rinsed their mouths once every week in school for 60 seconds with a placebo solution; Group II followed an identical procedure using a 0.2% neutral sodium fluoride solution (0.09%F). Group III rinsed their mouths once every day in school for 60 seconds using a 0.05% neutral sodium fluoride solution (0.023%F). The procedures were carried out under the classroom teacher's supervision for a period of three years. Baseline dental examinations, using the DMF surface index, were conducted in November 1977. An interim, follow-up examination was conducted in April 1979 and the final examination was conducted in May 1980.